Chromium-coating steel surfaces by the diffusion process



nited States atent O CHROMIUM-COATING STEEL SURFACESBY THE DIFFUSION PROCESS Gottfried Becker, Dusseldorf, and Fritz Steinberg,

Remscheid, Germany, assignors to DeutscheEdelstahlwerke Aktiengesell'schaft, Krefeld, Germany No Drawing. Application November 30,1953, Serial No. 395,204

6 Claims. (Cl. 117-107) The present invention relates to the chromium-coating of steel surfaces by the diffusion process for the purpose of combating corrosion. For promoting this end, steels have been proposed for facilitating the diffusion of the chromium into the surface, it being understood that diffusion takes place fundamentally in all steels. Thus, for example, it has been proposed to add one or more of the elements titanium, tantalum, niobium, vanadium, molybdenum, zirconium, manganese and aluminium to the steels in order to slow down or even to stop altogether the outward migration of the carbon because it was found that the opposing migrations of the carbon and the chromium may impair the formation of the chromium treated surfaces as produced by diffusion. In practice, steels which contain no such additional elements as well as those which do have been used. These elements are substantially equal to one another in their ability to slow down or suppress carbon migration, and the selection of the element to be used in any particular case depends on economic considerations or on whether it is intended to improve the mechanical properties of the steel, e. g., to facilitate the hardening thereof.

Chromium-rich surfaces have been produced by diffusion which have shown remarkable resistance to corrosive attacks of many varied kinds by reason of their high chromium content, which in some observed cases is as much as 50% at or near the surface. Nevertheless,

steels chromium-coated by diffusion sometimes fail to withstand corrosive attack, and this applies whether the additional elements are present or not and even with some modern diffusion processes, and recent practice has therefore been to dispense altogether with the said additional elements. This failure is remarkable because a chromium-steel casting, e. g., one having a 25% chromium content, does not fail to resist corrosion although its chromium content is a good deal lower than the chromium content aforesaid at least at or near the surface of the chromium coated steel.

It would not seem that a carbon content of the chromium treated surface produced by diffusion is the cause of the failure, especially as chromium castings having a carbon content of 1%, which is substantially higher than the carbon content which is generally present in surfaces produced by chromium diffusion do not exhibit such failures.

We believe that the failure in resisting corrosion of chromium diffused surfaces is due to corrosion which occurs along the grain boundaries (inter-crystalline corrosion). It is uncertain whether it is here a question of similar phenomena as are observed in the case of austenitic chrome-nickel steels after tempering treatment or whether diffusion phases of varyingly high chromium content occur which cause local potential differences and thus lead to corrosion. Whatever is the explanation corrosion does occur despite the comparatively high chromium content in the diffusion zone.

We have now found that the failure in question, can

Z ,7 91 ,5 l 7 Patented May: 7, 1957 process whilst at the same time being resistant at the surface to corrosive attack.

The action of the elements titanium, tantalum and niobium present in the steel to be employed in accordance with the invention is not affected by the addition of further alloy elements, such as for example manganese, silicon and molybdenum, which may be added to improve the mechanical properties of the steel.

Suitable steels for use in the process according to the invention are, for example, steels containing from 0.02 to 0.15% carbon. For economic reasons a carbon content of 0.1% is generally not exceeded.

A steel with carbon from 0.03 to 0.06% and titanium not exceeding 0.5% has been shown to be especially advantageous, the titanium content amounting to at least five times the carbon content.

Where niobium is employed, suitable steels for use in the process according to the invention, are for example steel containing: from 0.02 to 0.15% carbon and from 0.20 to 1.8% niobium, the niobium content being at least ten times the carbon content. A content of 0.1% carbon is generally not exceeded and the upper limit for the niobium content then should not exceed 1%. The steel preferably has from 0.03 to 0.06% carbon, as in the case of the steel containing titanium.

With the same carbon content, the amount of tantalum when this is used, should be at least eighteen times greater than the carbon content, and the tantalum content should not exceed 2%, and should preferably not exceed 1.5%.

If two or more of the elements titanium, niobium and tantalum are present together, as they may be, then they must be present in such quantities as together to fix the carbon content but it is no longer essential that there should be at least five times more titanium than carbon, or ten times more niobium than carbon or eighteen times more tantalum than carbon and the contents may be suitably adjusted accordingly. Where the carbon content is from 0.02 to 0.15 the sum of the said elements should not exceed 2.0% and where the carbon content is not more than 0.06% the said sum should not exceed 1.5%, the sum chosen being of course sufficient to combine with the carbon in a stable manner.

If the elements titanium, niobium and tantalum are all present in the steel, it is advantageous in the case of a carbon content of 0.05% to provide the following contents:

Approx. 0.l5% titanium Approx. 0.15 niobium Approx. 0.10% tantalum What we claim is:

1. The process of diffusion chromium treatment of steel to produce a chromium-rich surface resistant to corrosion which comprises diffusing the chromium into a steel containing carbon in an amount from about 0.02 to about 0.15 and at least one of the elements selected from the group consisting of titanium, niobium and tantalum in minimum quantities sufficiently greater than that necessary to stably fix the carbon present so as to give the chromium-rich surface the property of resisting intercrystalline corrosion, such minimum quantities be ing defined by the equations where Ti, Nb, and Ta stand for the percent of titanium, niobium and tantalum, respectively, present in said steel, 2:, y, and z represent fractional coefiicients, the sum of which is equal to 1, and C represents the total amount of carbon present in percent in the steel.

2. The process as defined in claim '1 wherein said steel contains from 0.03 to 0.06% of carbon, x has the value of l, and the maximum amount of titanium present is less than 0.5%. v

3. The process of claim 1 wherein said steel contains 2,791,517 U w p 7 carbon in an amount from 0.03 to 0.06%, y has the value of 1, and the amount of niobium present is less than 1%.

4. The process of claim I wherein said steel contains carbon in an amount from 0.03 to 0.06%, 1 has the value of l, and the total amount of tantalum is less than 1.5%.

5. The process as defined in claim 1 wherein one of the coefiicients x, y, and z has the value of O.

6. The process according to claim 5 in which the sum total of the said elements present in the steel does not 10 exceed 1.5 percent.

References Cited in the file of this patent UNITED STATES PATENTS Becker, et a1. May 7, 1946 

1. THE PROCESS OF DIFFUSION CHROMIUM TREATMENT OF STEEL TO PRODUCT A CHROMIUM-RICH SURFACE RESISTANT TO CORRISION WHICH COMPRISES DIFFUSING THE CJROMIUM INTO A STEEL CONTAINING CARBON IN AN AMOUNT FROM ABOUT 0.02 TO ABOUT 0,15%, AND AT LEAST ONE OF THE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF TITANIUM, NIOBIUM AND TANTALUM IN MINIMUM QUANTITIES SUFFICIENTLY GREATER THEN THAT NECESSARY TO STABLY FIX THE CARBON PRESENT SO AS TO GIVE THE CHROMIUM-RICH SURFACE THE PROPERTY OF RESISTING INTEERCRYSTALLINE CORROSION, SUCH MINIMUM QUANTITIES BEING DEFINED BY THE EQUATIONS 